This invention relates to "bag-type" filter apparatus, and more especially, to an improved combination tubular bag support and air diffuser means therefor.
As is generally known in the art, a bag-type filter apparatus generally includes a multiplicity of tubular filter bags mounted in a filter housing or "baghouse" with each filter bag having a tubular supporting frame or "cage" positioned therein so as to hold the filter bag in an open tubular configuration. Particulate-laden gas is directed into the filter housing and flows through the gas-permeable filter bags while the particulate material is filtered and retained on the exterior surfaces of the filter bags. Periodically, a reverse purge flow of air is directed into the outlet ends of the tubular filter bags for dislodging trapped particulate material from the filter bags and thus cleaning the filter bags.
One problem with this type of filter apparatus is that the cleaning operation is not very effective in removing particulate material from the filter bags. After the filter bags have been in use for a period of time, they become increasingly clogged with particulate material. This undesirably increases the pressure drop across the filter apparatus and significantly reduces its efficiency. In some filter applications, the inability to effectively clean the filter bags of the particulate material is so severe that it is necessary to frequently replace the filter bags. Also, as a result of the particulate material trapped on the filter bags and the resulting obstruction to air flow, the filter apparatus must be designed with a relatively large number of filter bags and with a relatively low ratio or air flow to filter area in order that the filter apparatus will remain serviceable as the filter bags become increasingly clogged with particulate material.
The presence of particulate material on the filter bags also causes abrasive wear of the bags and results in a shortened useful life. Typically, a filter bag will have a useful life of up to about two years. However, in severe applications, as for example where the particulate material is of a highly abrasive nature, the filter bags may wear out in a matter of weeks. Considering that the usual filter apparatus may contain many hundreds of filter bags, and that a single bag may cost from fifty to one hundred dollars, the cost of continually replacing the filter bags makes the operational cost of the filter apparatus quite high. Yet, in many applications the use of filtration apparatus is required by governmental air quality standards in order to limit the amount of particulate material in effluent gases.
In my U.S. Pat. No. 4,231,770, dated Nov. 4, 1980, I have disclosed an improvement in a bag-type filter apparatus which provides significantly more effective cleaning of a filter bag when a periodic reverse purge of air is directed into the filter bag. As disclosed in said patent, a hollow diffuser tube is provided interiorly of the conventional supporting cage provided within the filter bag for supporting and holding the filter bag in an open tubular configuration. The diffuser tube has one end thereof communicating with the outlet end of the filter bag and has perforations therein located for diffusing and distributing air throughout the filter bag when a periodic reverse purge of air is directed into the outlet end of the filter bag. The more uniform distribution of the purge of air throughout the filter bag provides considerably more effective cleaning of the filter bag than has heretofore been possible in a conventional bag-type filter system.
The diffuser tube of said patent is particularly suited for installation in a filter which is already equipped with a cage for supporting and holding the filter bag. In my aforementioned copending U.S. application, Ser. No. 06/076,942, I have provided within the filter bag a device which serves as a cage for supporting and holding the filter bag in an open tubular configuration and also serves for diffusing and distributing throughout the filter bag the reverse purge of cleaning air which is periodically directed into the filter bag.
More particularly, said copending application discloses a device in the form of an elongate tube extending longitudinally within a filter bag and having a series of spaced peripheral portions extending generally outwardly beyond intervening other portions of the tube and serving as a cage for engaging the surrounding filter bag and holding the same in a generally open tubular configuration substantially out of contact with the portions of the tube located inwardly therefrom. Those inwardly located portions of the tube have perforations therein located for diffusing and distributing air throughout the filter bag when a periodic reverse purge of air is directed into the tube. Thus, the device of said copending application serves as a combination bag support and air diffuser when installed within a conventional tubular filter bag.
It has now been determined that, at least in the case of some of the more common sizes of tubular filter bags now being used, e.g., tubular filter bags up to about ten feet (3.048 meters) long and up to about six inches (15.24 cm) in diameter, the cleaning of such filter bags is considerably enhanced by utilizing a perforate air diffuser tube which is substantially shorter than the filter bag, and is thus substantially shorter than the internal tubular support or "cage" for the filter bag. Stated otherwise, an exemplary perforate air diffuser tube was of a length substantially less than that of the filter bag, but was of a length greater than about one-half that of the filter bag. In the cleaning of filter bags in the sizes mentioned above, very good results have been achieved by providing an air diffuser tube of a length within about 60 to 75 percent of the overall length of the filter bag and the supporting frame or "cage" therewithin.